Semiconductor injection lasers are pumped, or energized, by arrays of laser diodes and are generally used for long distance communications both in space and on earth. These lasers rely on many individual laser diode assemblies, also known as laser diode sandwiches, positioned around a rod of semiconductor material. The individual laser diodes, or injection diodes, emit laser light into the semiconductor rod to cause it to laze, which is to say that the semiconductor rod is stimulated to emit a high power coherent light impulse. Common semiconductor rods used for this type of laser are made of materials such as neodymium doped yttrium aluminum garnet (Nd:YAG).
Semiconductor injection lasers have many advantages for communications usage. Their small size and mechanical stability have permitted their use in harsh environments which are unsuitable for other types of laser devices. In addition the injection lasers are capable of very high efficiencies when compared to other types of lasers and may be used in continuous or pulsed modes of operations.
Power output of the injection laser, produced at the semiconductor rod, is dependent upon the amount of light directed into the rod by the individual laser diodes. To achieve maximum efficiency, as much light as possible should be directed into the rod. In order to achieve this, the injection diodes are packed as closely together as possible in a manner which permits the diodes to conduct a large amount of current and produce large amounts of laser light. As a result, the diodes produce a large amount of heat and must be cooled. In this particular application heat dissipation from the laser diodes is critical both to the efficiency and the longevity of the individual diodes in the injection laser. Further, since the diodes are quite small and are packed so closely together the heat dissipation problem is aggravated by the need to extract a large amount of heat from a very small area.
This problem has been addressed in the past mainly by circulating a chilled liquid through a heat conducting material that is thermally coupled to the laser diodes. In other words, a refrigerant is used to draw the heat energy away from the laser diodes in a manner similar to the use of coolants in high speed computers where components are densely packed together and produce large amounts of heat. This solution greatly increases the cost and power consumption of the overall laser device and is often unsuitable for space applications. Further, the increase in system complexity implied by an active conductive cooling system acts to decrease system reliability.
As a result of these factors, systems have been proposed that extract heat from laser diode arrays purely by passive conduction. Passive conduction is particularly suitable for space systems in vacuum. Such passive systems make use of large copper and aluminum heat sinks that transfer heat from the laser diodes to a heat dissipation means. These types of devices, some of which have been moderately successful, require the heat from the injection diodes to travel from the diodes into a copper heat sink mounting plate and from there through a dielectric spacer and an aluminum heat extractor. From there the heat is directed to a spacecraft or aircraft radiator. This process of heat tranfer across so many interfaces causes a large thermal impedance. Large thermal impedances result in high laser diode operating temperatures which in turn result in diminished laser light output, low laser efficiency and shortened diode life.
All of the above factors act to limit the number of laser diodes that may be packed closely around the semiconductor rod and the amount of current that may be used to drive the diodes. It is an object of the present invention, therefore, to improve the heat transfer system between the laser diodes and the heat dissipation means so that an increased number of diodes can be operated at high power levels. This in turn would permit more efficient and more powerful injection lasers.
Another problem that presents itself with conventional cooling designs is a result of the thermal expansion mismatch between semiconductor diodes and the metal heat sinks they are generally mounted upon. In the case of copper, which is most common, the metal expands at nearly three times the rate of a typical semiconductor diode (per .degree.C.) and as a result, large mechanical stresses are imposed on the diode. Such stresses cause early failure of diodes due to cracking and dark line defects in the optical emitting areas.
It is therefore an object of the present invention to provide an improved mounting arrangement for the laser diodes that will reduce mechanical stresses due to thermal mismatch.